1. Field of Invention
The present invention generally relates to metal-gas cell batteries, such as metal-air cell batteries. More particularly, the present invention relates to mechanically rechargeable metal-air cell batteries.
2. Description of Related Art
More powerful and longer-lasting batteries are a high priority item for all countries seeking to replace hydrocarbon-fueled vehicles with smogless electrically powered vehicles. A great deal of research is thus presently focused on metal-gas cell batteries, such as zinc-air batteries. Zinc-air batteries have the highest theoretical specific energy content of all known battery types. Many problems, however, must be overcome before vehicles powered by zinc-air batteries, which are regarded as acceptable alternatives to vehicles burning hydrocarbon fuel.
All metal-gas cell batteries comprise a plurality of cells. Each of the cells has at least a gas cathode and a metal anode separated by a quantity of alkaline electrolyte and some form of mechanical separator sheet. During the operation of metal-gas cell batteries, a reactant gas, such as oxygen, reacts at each gas cathode to form hydroxide ions, and these hydroxide ions, in the alkaline electrolyte, react with metal anode material at each metal anode. The process creates an electrical potential between each gas cathode and each metal anode. When the cells are connected in series, the combined electrical potential of all cells is considerable and can be used as a source of electrical power. As can be seen, however, the operation of the battery gradually depletes the available metal anode material. Therefore, the battery has to be periodically recharged.
Metal-gas cell batteries can be recharged either electrically or mechanically. Electrical recharging can be easily adapted to the existing power networks, but the service life of the electrically rechargeable metal-gas battery is markedly limited. Moreover, an electrically rechargeable metal-gas battery requires a bifunctional or an additional gas diffusion electrode. Due to the need for such a bifunctional or additional gas diffusion electrode, the battery is unduly heavy, bulky and complicated.
Accordingly, the current preferred recharging mode for metal-gas cell batteries is mechanical refueling, whereby the spent metal anode is physically replaced with a fresh metal anode. Mechanical refueling can be accomplished in two ways. When the metal anode comprises metallic pellets or powder suspended within the electrolyte, the spent metallic pellets or powder is pumped from the cell and fresh metallic pellets or powder is pumped into the cell. U.S. Pat. Nos. 3,981,747, 5,006,424, 5,434,020 and 5,558,947 disclose attempts to use zinc particles or pellets as anodes.
An even simpler method of mechanical refueling is possible if the metal anode is a rigid structure, for example, made of a conductive support packed with zinc powder. The spent metal anode is removed and a fresh metal anode is reinstalled into the cell. This refueling method is generally employed because of its theoretical, construction, maintenance and operation simplicity. U.S. Pat. Nos. 3,513,030, 5,208,526, 5,318,861, 5,366,822, 5,418,080, 5,447,805, 5,753,384, 5,904,999 and 6,057,052 all disclose various methods of mechanically refueling metal-gas cell batteries by replacing the spent rigid anode structure. Each of the patents listed is incorporated herein by reference in its entirety.
One problem with such conventional metal-gas cell batteries is that the rigid anode structures are difficult to remove from and insert into the cell. In a conventional cell where the housing of the cell is wholly rigid, clearances for the removal and reinsertion of such anodes are generally very small. The gas cathodes and separator sheets are often abraded during the removal and reinsertion of the anodes. U.S. Pat. Nos. 4,389,466 and 4,560,626 disclose an attempt to solve this problem. However, in these patents, the total contact area between the cone-shaped current collectors and the metal anodes of the metal-gas cell batteries is not sufficiently large for large currents. Moreover, pinpoints on the current collectors often make the insertion and extraction of the metal anodes very difficult. Another attempt to solve this problem is disclosed in U.S. Pat. No. 5,286,578. In this patent, a collapsible electrochemical cell made by “a flexible plastic material” is suggested to satisfy its collapsible design. No detail of the flexible plastic material is disclosed. However, such housing system is fragile and cannot withstand repeated refueling. Other wholly flexible housing systems are disclosed in U.S. Pat. Nos. 5,415,949 and 5,650,241. Such housing systems are unduly complex and are therefore expensive to manufacture, maintain and operate.
U.S. Pat. Nos. 4,389,466 and 4,560,626 disclose a soft bladder to press the zinc anode against the multi-points and cone-shaped current collector. These designs have many problems and are not discussed here. In fact, no commercial product on the market comes even close to the designs described in these patents. Another problem with metal-gas cell batteries, which are mechanically refueled by physical replacement of a rigid anode structure, is the frequent leakage of the alkaline electrolyte. In most prior art designs, the housing of the metal-gas cell is usually opened at the top. The opening is sealed during operation by an elastic sealing element disposed between the cell housing and a protruding portion of the anode assembly. This protruding portion of the anode assembly is universally used in such designs for electrical connection to other battery electrodes. Moreover, it is common to provide one or two small breathing holes along the uppermost portion of the cell proximal to the protruding portion of the anode. However, alkaline electrolyte tends to creep up the anode and out of the cell along the protruding portion of the anode. Also, alkaline mist continuously escapes through the breathing holes. Such leakage and mist can cause rapid oxidation of the conductors of the anode and the air cathode. Oxidation dramatically increases the electrical resistance between the contact surfaces and therefore results in a marked loss of battery power. Moreover, the continual leakage of alkaline electrolyte and electrolyte mist makes the battery difficult to use in any kind of environment where oxidation of metallic items outside the battery is a problem. Finally, any upset of the battery during handling or operation will cause copious leakage of alkaline electrolyte out of the battery.
As a matter of fact, secondary zinc-air fuel cells or batteries, no matter whether mechanically refuelable or electrically rechargeable, have not been manufactured on a large scale as a commercial product. Only primary zinc-air button cell and zinc-air battery, which are not rechargeable, for navigation lamps can be found on the market at present. This is because no one has yet solved the problem of the separator.
All the secondary zinc-air batteries having zinc electrode suffer from a short service life because the batteries are short-circuited by zinc dendrites growing from the zinc electrode during recharging. The sharp zinc dendrites stab the separator like needles and bridge the zinc anode to the air cathode. Therefore, short circuits often occur, Batteries like Ni—Zn battery, Ag—Zn battery and the electrical rechargeable zinc-air fuel cells have the same serious problems due to frequent recharging. Theoretically, the mechanically refuelable secondary zinc-air fuel cell should not suffer from this problem, as it is not electrically recharged. However, the mechanically refuelable zinc-air fuel cells do suffer this problem, although with lesser frequency. Even at a lesser frequency, it is still a serious problem, as when even one cell in a multi-cell module is short-circuited, the whole module fails.
The principle by which unpredicted zinc dendrites occasionally grow and cause short circuits is not clear yet, but may be due to the following reason. During the discharge of the uneven density of the zinc powder distribution on the anode plate, the electrical potential is different on different location of the zinc electrode. As a result of reducing zincates in the alkaline electrolyte to deposit metal zinc on the zinc anode surface, the zinc dendrites are formed at lower potential locations of the zinc electrode. These dendrites rapidly span the narrow gap between the anode and the cathode to short the cell. Therefore, the previous art designs either cannot avoid occasional short circuits or sacrifice significantly the performance of the cell.
To avoid the zinc dendrites causing short circuits, in the traditional zinc-air battery used for powering a navigation lamp, the zinc electrode is usually wrapped in multiple layers of separator paper to enhance resistibility thereof stabbing by zinc dendrites. Additionally, the distance from the air cathode to the zinc anode is enlarged to about 10 mm or more, so that the zinc dendrites cannot grow long enough to reach the air cathode. As the result of increasing the electric resistance of the multiple layers of separator paper and the thickness of the electrolyte, the internal resistance of the cell is increased, too. Hence, this kind of zinc-air battery can only deliver low power; it is enough to power a navigation lamp or a communication equipment unit, but not adequate to power an electric vehicle.
Mechanically refuelable secondary zinc-air fuel cells are expected to have a service life that is a few hundred times longer than the disposable zinc-air fuel cells. The separator is too expensive to be renewed during every refueling; the separator has to be reusable over the whole service life of the zinc-air fuel cells. U.S. Pat. No. 5,418,080 discloses a 400-mesh fabric separator bag employed in a zinc-air fuel cell. This separator bag is made of polypropylene fabric or polymeric amide and is expected to be strong enough to work in an alkaline electrolyte for several years. But the separator bag needs to be slipped onto the zinc anode plate and has to be washed after every discharge. Otherwise, zinc oxide residue tends to block the pores of the fabric as well as the transfer of ions, which ions would otherwise penetrate the pores of the fabric to transfer electricity between the cathode and the anode. As a result, the output power is lowered from time to time. U.S. Pat. No. 5,431,823 also discloses a specially designed tool for washing the separator bag. Even with this specially designed washing tool, the separator bags have to be manually slipped onto and removed from the washing tools. Besides, the separator bags have to be manually slipped onto and removed from the zinc anodes as well. If the huge quantity of anodes and separator bags when hundreds of thousands of zinc-air fuel cells are in use is considered, this labor-intensive processing is obviously too expensive to be a real commercial solution, even in developing countries.
Furthermore, although by visual observation, the 400-mesh fabric is a fine and close texture, its openings are too large to prevent the zinc dendrites from passing through. Therefore, the short circuit caused by zinc dendrites cannot be avoided for sure.
Kummrow's zinc-air battery uses fabric as the separator bag, too. Slightly differing from U.S. Pat. No. 5,418,080, it uses a thicker fabric. In order to prevent short circuits caused by zinc dendrites, a polypropylene box with big holes on its two major surfaces is used to envelop the zinc anode and its separator bag. Therefore, it entails not only the expensive, labor-intensive operation of changing and washing the separator bags, but also the additional changing and washing the polypropylene boxes. Furthermore, the gaps between the zinc anodes and the air cathodes are increased to about 10 mm. This construction may not suffer from short circuits caused by zinc dendrites, but the power output is significantly reduced. It may suitable for low power use, but definitely is not suitable for high power requirements, such as electric vehicles.
From the commercial point of view, the expensive labor-intensive operation of changing and washing the separator bags has to be eliminated; the separator has to be installed permanently in the zinc-air fuel cell. Consequently, none of the existing constructions solves the problem caused by zinc dendrites short circuits.
Another serious problem arises when the separator is permanently installed in the zinc-air fuel cell. The pores of the separator are passages for the hydroxide ions. If the pores are blocked by zinc oxide, the hydroxide ions are blocked, too. Hence the zinc anode will be hungry from lack of hydroxide ions, and the power output will decrease over time as the openings of the separator are increasingly blocked. Even when the discharging current is turned off, the pores of the micro-porous separator are plugged by the precipitation of zincates to zinc oxide from the electrolyte.
The separator is one of the keys to the performance and durability of the secondary zinc-air fuel cells and batteries. The separator's ability to control the exchange of ions plays a limiting role in determining maximum power to weight ratio; this is especially important for the zinc-air fuel cells for powering electric vehicles.
Attempts to avoid the dendrite-shorting problem using metal or metal oxide as barrier layers are illustrated in U.S. Pat. Nos. 3,539,396 and 4,298,666. Nickel powder is commonly used in these patents because it reacts with the zinc dendrites as a micro cell to prevent the continuous growing of the dendrites. Separators selected from the above patents could be the solution to the separator problem for zinc-air rechargeable battery, although they are expensive.
For mechanically refuelable zinc-air fuel cells, no matter whether its discharged anode has to be replaced or recharged in another recharging cell, the separators with metal or metal oxide barrier layer are not necessary. Separators selected from the following patents may be good enough for mechanically refuelable zinc-air fuel cells. U.S. Pat. Nos. 4,154,912, 4,272,470 and 6,033,806 disclose a graft polyvinyl alcohol separator, which may be effective to avoid dendrite short-circuiting.
In U.S. Pat. No. 4,359,510, a novel structure of separator is disclosed. A hydrophobic micro-porous non-woven web is first treated with a wet agent and then coated with cellulose on both sides. This manufactured separator has a low ionic resistance, good hydrophilic ability, dendrite-shorting resistance, and pore-plug resistance.